The invention relates to a display unit for object illumination data in a single lens reflex camera, in particular such a camera incorporating an electronic shutter.
Referring to FIG. 1 to describe the background of the invention, there is shown a single lens reflex camera including a taking lens 1 and a film surface 2. Intermediate the lens 1 and the film surface 2 is disposed a movable reflecting mirror 3 at an inclination of 45.degree. to permit the light which passes through the taking lens 1 to enter a finder light path in order to focus the image of an object being photographed on a focussing screen 4 located at a position conjugate to the film surface 2 with respect to the movable reflecting mirror 3. The image on the screen 4 is viewed through a condenser lens 5, Pentadachprism 6 and an eyepiece 7, thereby enabling the same image as the one being photographed to be observed through the finder.
In order to determine proper values of the exposure factors such as shutter speed, diaphragm aperture or the like automatically in such a camera, a light receiving element 8 is located within the light path of the finder to effect photometry of the light from an object being photographed. This is known as a TTL (through the lens) photometry system.
However, when an electronic shutter is incorporated into such a camera, the incidence of light onto the light receiving element which determines the exposure period is interrupted to thereby preclude a proper determination of the exposure period by the electronic shutter, because when taking pictures, the reflecting mirror 3 is moved out of the path of the photographing light closing off the finder light path. As a consequence, when using an electronic shutter in a single lens reflex camera, a memory circuit is used to store the object illumination data obtained before the movable reflecting mirror 3 is moved out of the path of the photographing light, and the stored data is subsequently used to determine the exposure period.
The means for storing the illumination data may be analogue in nature where the light receiving element comprises CdS, the voltage from the CdS element being compressed logarithmically with respect to the light quantity and stored across a capacitor and the capacitor voltage being again expanded logrithmically in the up position of the mirror to drive the shutter. Alternatively, the storing means may be constructed in a digital manner by using an arrangement such that the light quantity which is incident on the light receiving element as an analogue quantity is counted in terms of pulses, which pulses are stored in a memory circuit capable of storing a digital quantity in terms of a combination of resistances or current values, the stored data being read out through a digital-to-analogue converter (referred to hereinafter as D-A converter) in the up position of the mirror to drive the shutter.
It is desirable that the illumination data thus obtained be displayed in some form to a photographer. With a single lens reflex camera incorporating an electronic shutter, the exposure period is automatically determined and a photographer can not know how fast the shutter operated. It will be advantageous for the benefit of affording the photographer with information concerning the parameters of taking pictures if the object illumination data, such as the exposure period which is determined automatically, can be displayed. It is also desirable that such ilumination data be displayed within the light path of the finder in a single lens reflex camera.
While there have been some cameras which provide a display of the exposure period, they involve the use of a meter as display means. However, such a display provided by the pointer of a meter has a disadvantage in that such a display can only be read with great difficulty. It will be appreciated that the illumination data can be readily read if the display is provided by a display element 9 such as lamp as shown in FIG. 1.
However, the placement of a lamp display 9 within the light path of the finder may cause an error in the illumination data because the light receiving element 8 also measures the light quantity from such a display element. Such an error will be pronounced particularly when taking a picture of an object which is under dim illumination. Referring to FIG. 1, it will be appreciated that the light Bv from an object being photographed passes through an optical system including the taking lens 1, movable reflecting mirror 3, pentaprism 6 and the like before reaching the light receiving element, so that a considerable amount of attenuation occurs, the resulting reduction in the light er dim illumination.
There is another problem involved with such a positioning of the display. When a bright display takes place within the finder, the pupil of a man will respond to the brightness thereof, with the result that a dark image of the object which is focussed on the focussing screen 4 within the light path of the finder can not be clearly recognized or the luminescent display is viewed as glaring.